Annual measurements of the Forced Expiratory Volume in One Second (FEVI), the Forced Vital Capacity (FVC), the FEVI/FVC%, and the Maximal Mid-Expiratory Flow Rate (MMEF) have been made, primarily by three technicians, over a six year follow-up period of 646 middle-aged male participants in the Multiple Risk Factor Intervention Trial (MRFIT), Pittsburgh Center. Much attention has been paid to the quality of the spirometric testing over this period of time, and indices reflecting the quality of the pulmonary function testing have been developed and can be used in the analysis of these data. A cross-sectional analysis of the spirometric indices by smoking status and amount smoked at Baseline has been completed. Using the annual lung function measurements, the relative rates of decline of the spirometric indices among the never smokers, ex-smokers, and continuing smokers can be examined. Since the MRFIT study is a randonized clinical trial designed to assess the effects of risk factor reduction on heart disease incidence, smoking cessation is emphasized, resulting nationally in a high proportion of ex-smokers: 47% of the Special Intervention group and 25% of the Usual Care control group smokers had quit smoking by the Fourth Annual Examination. Since the follow-up is fairly lengthy (six years), complete (93% of men followed through six years), of good quality, and with annual spirometric testing, the different FEV1 decrements observed by Fletcher et al (1) for never, ex-,. and current smokers, with and without airways obstruction, can be validated, and the changes in the MMEF and the FEV1/FVC%, not included in the Fletcher study, can also be analyzed for this healthy working population. The MMEF longitudinal changes may be of particular interest, since the MMEF is thought to reflect early obstruction in the small airways (2), and its decline over time has not been well documented in persons with minimal or no respiratory disease. Except for Fletcher's cohort, no other large healthy working population has been followed for as long as six or seven years, with at least annual lung function measurements. The analyses desinged for the Pittsburgh MRFIT data can subsequently be extended to include several other MRFIT centers with good quality measurements, increasing the sample size by several times.